Rheology and structures of aqueous gels of diblock(oxyethylene-oxybutylene) copolymers with lengthy oxyethylene blocks

Aqueous solutions of diblock copolymers E96B18, E184B18, E315B17 and E398B19 (E=oxyethylene unit, B=oxybutylene unit) were investigated by rheometry and small-angle X-ray scattering (SAXS). Storage (G') and loss (G ") modulus and yield strength (sigma(y)) were used to define hard- and soft-gel phases in experiments which covered the concentration range 2-14 wt.% copolymer. Values of G' correlated with those of yield strength, the ratio G'/sigma(y) being ca. 0.03. SAXS was used to explore hard-gel structures, and to confirm hard-gel/soft-gel boundaries. The sol/soft-gel boundary was identified as a percolation threshold. The effect of an increase in E-block length was to move the gel phases to lower concentrations without changing the pattern of their behaviour. In this respect, the critical conditions for hard-gel formation (c*, T*) served as parameters for a 'universal' phase diagram.